HIIT to improve anabolism and hypertrophy....not really. He is connecting the dots of theoretical research with a yellow colored pencil.

This is part true.

Trouble is, the only way to increase the number of intracellular nuclei is to perform strength-endurance training—which will actually make the muscle smaller.

True - Local endurance training will cause a decrease in the fiber size and an intraconversion of twitch properties.

False. Myofibril nuclei increase when satellite cells (myogenic stem cells) fuse with a damaged muscle fiber. This muscular damage will occur with nearly any type of overload so long as muscle damage, inflammation, and MGF expression occur.

This process typically shuts down the mTOR pathway during steady-state cardio, but research shows that this doesn’t happen during HIIT.

This also isn't entirely true. Depending on the duration and work to rest ratio of HIIT, it can activate AMPK (good for body composition purposes) which will temporarily reduce mTOR expression.